Dave Barnes

Antarctic octopus sounds a warning over sea-level rise

Turns out that looking at DNA can tell us a lot about what happened 125,000 years ago.

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Turquet’s octopus lives in the Southern Ocean all around Antarctica, where it spends its life mostly on the seafloor, not moving far from the place it hatched. Its skin is smooth, pale and freckled, its body not much bigger than a human hand.

Like other octopus species, Turquet’s octopus only breeds once at the end of its life, dying soon after. But unlike other octopus species, it doesn’t have a larval stage which can drift on ocean currents to settle in new areas. Its young crawl on the seafloor and don’t travel far.

Now, its stay-at-home nature has been key to a major scientific discovery.

Populations of Turquet’s octopus are dotted around several regions of Antarctica, including the Amundsen Sea, the Weddell Sea and the Ross Sea, giant bays on different sides of the West Antarctic ice sheet. Octopuses living in these seas are completely isolated from each other and never meet. Even though adults venture further afield than hatchlings, the next bay over is just too far away.

But genetic research shows that these octopus populations used to meet up—and not only meet up, but hook up. There’s only one way they could have done so: if the ice sheet separating them didn’t exist.

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Looking out towards the West Antarctic ice sheet earlier this summer, it’s difficult to imagine it not being there. Its flat icescape stretches as far as the eye can see, but if it vanished, all that would remain are islands separated by open sea.

West Antarctica is an archipelago, its islands wrapped in ice. Because the ice sheet rests mostly on the seafloor, it’s more vulnerable to being melted by warming oceans, in contrast to Antarctica’s larger eastern ice sheet which sits on top of a mountain range.

For decades, scientists have been trying to pinpoint the last time the West Antarctic ice sheet collapsed. It holds enough ice to raise global sea levels by three to four metres, if it were to melt completely.

According to the octopus DNA, the Amundsen, Weddell and Ross Sea octopuses were interbreeding during a period known as the Last Interglacial, around 125,000 years ago. This suggests the ice sheet had vanished and the sea level was higher, creating passages between the three seas—enough of a shortcut for the octopuses to meet up.

“These connections could have only happened if the ice between the three basins had melted or retreated enough for the octopus to migrate directly between the regions,” says Sally Lau, an evolutionary geneticist at James Cook University in Australia and the lead researcher of the octopus study.

During the Last Interglacial, the planet was about 1.5°C warmer than pre-industrial times due to natural variations in Earth’s orbit—similar to temperatures we’ll likely reach within a decade due to climate change. The octopus findings suggest that the West Antarctic ice sheet could reach that tipping point even under the Paris Agreement targets of limiting warming to 1.5 to 2°C.

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Scientists issued the first warnings that even a relatively small temperature rise could trigger melting of the West Antarctic ice sheet back in 1978. Today, the region is Antarctica’s biggest contributor to global sea-level rise, with most of the ice loss coming from glaciers in the Amundsen Sea, where warming oceans are eroding submerged portions of the ice sheet from below.

“It’s concerning because once the temperature threshold is crossed, the process of melting will carry on for centuries into the future,” says Tim Naish, an earth scientist at Victoria University of Wellington’s Antarctic Research Centre who contributed to the study.

“It cannot be stopped simply by returning global warming to below 1.5°C, because the ocean and the ice sheet hold onto their heat for decades to centuries and the system responds very slowly.”

Looking over the Amundsen Sea from Marie Byrd Island.

Climate models deliver contradictory projections about whether the West Antarctic ice sheet is likely to collapse completely or only partially. “Using the period of the Last Interglacial as a crystal ball for the future is challenging,” says Nick Golledge, a Victoria University climate modeller.

“A warming of 1.5°C might not be disastrous if it was a short-lived overshoot. But there are all sorts of feedbacks that we don’t really know or understand, and which aren’t properly included in climate and ice-sheet models.”

Geologists already have evidence that the West Antarctic ice sheet collapsed during a period in Earth’s history known as the Pliocene, some three million years ago, when it was 3 to 4°C warmer and carbon dioxide levels in the atmosphere were similar to today, above 400 parts per million. (The geological clues are in the form of sediment layers deposited on the ocean floor below the Ross Ice Shelf and around Antarctica.)

The octopus DNA data match these earlier events. But they also point to the much more recent ice sheet collapse during the Last Interglacial. Geological evidence for this time window is sparse, but an international effort is underway to retrieve sediment cores from the fringe of West Antarctica, where the ice sheet begins to float, and becomes the Ross Ice Shelf. These sediments may show layers of algal detritus, suggesting open-ocean conditions, or grainy mud, implying an ocean covered by ice. Either way, they would deliver more direct evidence about what happened during the Last Interglacial and how sensitive the ice sheet is to warming of 2°C.

Using animal DNA as a proxy to track past climate conditions is a new approach, says Jan Strugnell, also at James Cook University. “It is a very exciting space to work in. Animals respond to environment changes and their responses are recorded in their DNA.”

Turquet’s octopus turned out to be an ideal animal. It’s already well studied, including its mutation rate and how long it takes to breed—which is necessary to figure out the timing of interbreeding that took place. But it took a major global effort and 32 years to collect enough specimens to represent the populations all around Antarctica.

Sally Lau’s focus is now on piecing together the climate history of other parts of Antarctica, using the same methods. While we’re used to looking back in time via geology, biology also contains clues to the deep past, says Lau: “DNA is like a time capsule.”

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